1. Field of the Invention
This invention relates to digital magnetic read/write transducers particularly with respect to thin film heads having disparate pole pieces to compensate for asymmetries inherent in a high density magnetic recording system.
2. Description of the Prior Art
Magnetic read/write transducers have undergone a rapid evolution in recent years in order to keep abreast of the computer industry's need for ever increasing bit storage densities. The application of photolithographic techniques, used for years to fabricate microelectronic components, has enabled the manufacture of thin film heads, which can be more accurately aligned than conventional ferrite heads, and have high permeability at frequencies up to 100 megahertz. In addition, the greatly reduced volume of magnetic material in these thin film heads results in lower subceptibility to electromagnetic noise, while the thinner pole tips allow finer resolution than can be achieved with conventional heads. However, even with today's thin film head, bit storage density is limited, as will be shown, by the nature of the recording medium, be it disk, tape, floppy disk, card, or drum.
Magnetic recording is a sequential process. A magnetic head is used to magnetize small areas or spots on the surface of a medium where the resulting magnetic flux at each spot points in one of two opposite directions according as a zero or as a one is being stored. The sensing of a stored bit of information is accomplished by means of a relative motion between the surface and the same or a similar head, such that changing flux lines pass through the head and induce a voltage, the polarity of which indicates whether a 0 or a 1 is being detected. The bit positions, or data cells, are normally arranged in tracks along the surface of the medium so that the bits in any one track pass sequentially in the vicinity of the writing or reading head. It is also common practice to record new information over the old information and to depend on the writing process for erasing the old information. Thus, to some extent the magnetic history of each data cell in the medium, and the demagnetizing field produced by apparent magnetic poles within the medium contribute to the total magnetic field surrounding the gap. As data cell space is made smaller and smaller, in pursuit of ever increasing bit storage density, the magnetic properties of the recording medium itself begin to have a substantial effect on the fringe field surrounding the head.
It is understood today that the main limitation on bit density is imposed by the writing-demagnetization process, and in particular by the recording medium interaction with the process. This interaction can be expressed in terms of two phenomena: linear distortion resulting from intersymbol interference and characterized by a non-constant read back amplitude and symmetrical pulse spreading; and non-linear distortion due to the inherent hysteresis properties of the magnetic media and demagnetizing fields associated with the magnetic history of the medium, characterized by asymmetrical, downstream, peak shifting and amplitude anomalies. It has been found that for bit densities beyond 3,000 flux changes per centimeter, non-linear distortion emerges as the dominant factor. While the linear distortion can be adequately minimized using electronic pulse shaping or linear spectral equalization networks, non-linear distortion has not heretofore been so easily controlled. Indeed, the non-linear nature of magnetic hysteresis properties defies simple compensation with electronic circuitry.